Amino acid derivatives used as pharmaceutical substances

ABSTRACT

The invention relates to a method for improving bioavailability of pharmaceutical substances and for allowing the pharmaceutical substances to permeate the blood-brain barrier, the pharmaceutical substances having at least one or more amidine, guanidine, N-hydroxyamidine (amidoxime) or N-hydroxyguanidine functions. The invention also relates to medicaments containing the correspondingly modified pharmaceutical substances.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of International Application No.PCT/EP2009/051162, filed Feb. 2, 2009, which was published in the Germanlanguage on Aug. 6, 2009, under International Publication No. WO2009/095503 A2 and the disclosure of which is incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to the use of an amidoxime amino acidester in a method for improving the bioavailability and enabling theblood-brain barrier crossing of medicinal substances, which have atleast one or more amidine functions, guanidine functions,N-hydroxyamidine (amidoxime) functions or N-hydroxyguanidine functions,and drugs containing correspondingly modified medicinal substances.

BACKGROUND OF THE INVENTION

N-hydroxyamidines (amidoximes) and N-hydroxyguanidines represent knownprodrug principles for increasing the oral bioavailability of amidines[Clement, B. Methoden zur Behandlung und Prophylaxe der Pneumocystiscarinii Pneumonie (PCP) und anderen Erkrankungen sowie Verbindungen undFormulierungen zum Gebrauch bei besagten Methoden, P 432-444.4, 1993]and guanidines.

Pharmaceutical preparations containing an active agent having one ormore amidine or guanidine functions exhibit almost no pharmacologicaleffect in oral application. The precondition for a therapeutical effectof an active agent after oral administration is represented by itsresorption from the gastrointestinal tract. The most important mechanismof such an effect is passive diffusion. The degree of resorption by wayof passive diffusion is dependent on the lipophilicity and thus also theacidity and basicity of the active agent.

Highly basic compounds such as amidines and guanidines are present inthe stomach (pH 1) and small intestine (pH 6.4) in an almost completelyprotonated form. A resorption after oral administration, which requiresthe passing of lipid bilayers of the membranes of the gastrointestinaltract, therefore only occurs to a very low degree.

Another problem in the medication of many diseases is the necessity tocross the blood-brain barrier. The blood-brain barrier is an effectivebarrier with regard to the resorption of substances into the brain. Itensures the selective uptake and prevents the penetration of substances.Furthermore, the blood-brain barrier does no only act as a physical butalso as an enzymatic barrier. Various processes are involved in thepenetration of substances into the brain. As compared to otherindications, only few drugs are marketed, which develop their effect inthe central nervous system (CNS). The greater part thereof reaches theCNS by diffusion. Diseases like epilepsy, chronic pain or depressionsare treated this way. Other serious functional disorders such as braintumors or amyotrophic lateral sclerosis yet cannot be treated in thisway today [PARDRIDGE, W. M. NeuroRx 2005, 2, 3-14]. For being able tocross the blood-brain barrier by way of passive diffusion, a substanceis required to be lipophilic, have a lower molecular weight than 400-500Da, and be present in an uncharged state. For resorbing specificallysmall molecules such as glucose or amino acids, various transportersystems such as nucleoside transporters, influx and efflux transportersfor organic anions, glucose transporters, peptide transporters, andamino acid transporters are expressed on the blood-brain barrier [TAMAI,I.; TSUJI, A. J. Pharm Sci 2000, 89, 1371-1388 and DE BOER, A.; VAN DERSANDT, I. Annu Rev Pharmacol Toxicol 2003, 43, 629-656]. Largermolecules such as insulin or iron-containing transferrin are resorbedvia the receptor-mediated transport. In this case, the insulin andtransferrin receptors particularly play an important role [DE BOER, A.;VAN DER SANDT, I. Annu Rev Phamacol Toxicol 2003, 43, 629-656;PARDRIDGE, W. M. Mol Interv 2003, 3, 90-105]. Taking the amino acidL-dopa as an example, one has used a prodrug principle for thewater-soluble catecholamine dopamine to be capable of passing theblood-brain barrier. The transport is performed by the amino acidtransporter LAT1 (large neutral amino acid transporter). After thepassage, the decarboxylation into dopamine takes place [PARDRIDGE, W. M.Mol Interv 2003, 3, 90-105].

Diamidines are used as antiparasitic agents against malaria,pneumocystis jiroveci (previously carinii) pneumonia, trypanosomiasis(African sleeping sickness), and leishmaniasis [WERBOVETZ, K. Curr OpinInvestig Drugs 2006, 7, 147-157]. Particularly in developing countries,these diseases represent a serious problem involving high mortalityrates.

Three diamidines to be applied parenterally are on the market.Pentamidine (Pentacarinat®) has been used in the early stage of theAfrican sleeping sickness already for 60 years. Efficiency is no longerprovided in the 2^(nd) stage of the African sleeping sickness, themeningo-encephalitic stage, since the blood-brain barrier cannot bepassed successfully. As a result, highly toxic arsenic compounds must beadministered. There is a lack of medicinal substances which areefficient in the 2^(nd) stage of the African sleeping sickness.

It is likely that all of the active agents, which have an amidine or aguanidine as a functional group, exhibit an insufficient resorption inthe oral application, if they can only be resorbed by passive diffusion.

The N-hydroxylated derivatives such as amidoximes, and theN-hydroxyguanidines, due to the introduction of the oxygen atom, exhibita lower basicity. Under physiological conditions, they are not presentin a protonated form. Benzamidoxime represents a model compound for manymedicinal substances containing an amidoxime function [Clement, B., DrugMet Rev 2002, 34, 565-579].

Pentamidine and diminazene represent diamidines and are not resorbedafter oral application. They were therefore transferred into amidoximeprodrugs. (DE 10 2006 034 256.9).

The example pentamidine demonstrates that the transfer into thepentoxime ester also entails a reduction of the solubility. Thisprobably is also the reason for the bioavailability of pentamidine notto reach one hundred percent after oral application of the pentoximeester [Clement, B.; Bürenheide, A.; Rieckert, W.; Schwarz, J.,ChemMedChem 2006, 1, 1260-7].

Furthermore, the reduced water solubility induces the disadvantage thatan administration of the medicinal substance is no longer possible byinjecting aqueous solutions. This is a problem, particularly when anoral administration is out of question.

It is therefore the task of the present invention to increase the watersolubility and thus also the oral bioavailability of substances, whichhad been transferred into amidoxime prodrugs or N-hydroxyguanidineprodrugs, and/or thus to enable the passing of the blood-brain barrier.

The task is solved by methods according to embodiments of the invention,such as those methods having the features of the independent claim, andthose methods having the additional features of the dependent claims,which indicate advantageous arrangements of the invention.

BRIEF SUMMARY OF THE INVENTION

In one general aspect, the present invention relates to a methodcomprising using a partial structure forming formula (I), formula (II)or formula (III),

as a constituent of the overall structure of a prodrug for a medicinalsubstance, wherein R1 is selected from the group consisting of hydrogen,an alkyl radical, an aryl radical, and one selected from the groupconsisting of the following:

and the salts thereof.

A method according to an embodiment of the present invention comprisesusing an amidoxime amino acid ester of the formula (I), anN-hydroxyguanidine amino acid ester of the formula (II) or an amidoximeprolinyl ester of the formula (III),

wherein R1 is selected from the group consisting of hydrogen, an alkylradical, an aryl radical, and one selected from the group consisting ofthe following:

and the salts thereof, as a substitute for one or more amidinefunctions, N-hydroxyamidine (amidoxime) functions, guanidine functionsor N-hydroxyguanidine functions of a prodrug for a medicinal substancefor improving the solubility, bioavailability and/or capacity of themedicinal substance to pass the blood-brain barrier.

Another general aspect of the present invention relates to a prodrugcomprising a partial structure having the formula (I), (II) or (III),

wherein R1 is selected from the group consisting of hydrogen, an alkylradical, an aryl radical, and one selected from the group consisting ofthe following:

and the salts thereof, the prodrug is a prodrug for a medicinalsubstance, the medicinal substance is selected from the group consistingof protease inhibitors, DNA-intercalating compounds, RNA-intercalatingcompounds, inhibitors of viral enzymes and N-methyl-D-aspartate receptorantagonists.

In an embodiment of the present invention, the protease inhibitor is athrombin inhibitor, an inhibitor of factor Xa, Factor VII or all of theproteases of the coagulation cascade, or a matriptase inhibitor.

In another embodiment of the present invention, the DNA-intercalatingcompound or RNA-intercalating compound is pentamidine, diminazene orisometamidium.

In yet another embodiment of the present invention, the inhibitor ofviral enzymes is a neuraminidase inhibitor.

According to an embodiment of the present invention, the medicinalsubstance is configured for the prophylaxis and therapy of visceraland/or cutaneous leishmaniasis, trypanosomiasis, the 2^(nd) phase oftrypanosomiasis, or pneumonia caused by pneumocystis carinii, forinhibiting the growth of malign tumors, for inhibiting bloodcoagulation, for blood pressure reduction, for neuroprotection, and forcombating viral infections including influenza and HIV infections.

Other aspects, features and advantages of the invention will be apparentfrom the following disclosure, including the detailed description of theinvention and its preferred embodiments and the appended claims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe invention, will be better understood when read in conjunction withthe appended drawings. For the purpose of illustrating the invention,there are shown in the drawings embodiments which are presentlypreferred. It should be understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown.

In the drawings:

FIG. 1 is a graphic representation of the plasma levels of benzamidinein rats after the oral application of valbenzamidoxime;

FIG. 2 is a flow chart depicting the metabolism of the diminazene valineester; and

FIG. 3 is a flow chart depicting the metabolism of the pentamidinevaline ester.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to one of ordinary skill inthe art to which this invention pertains. Otherwise, certain terms usedherein have the meanings as set in the specification. All patents,published patent applications and publications cited herein areincorporated by reference as if set forth fully herein. It must be notedthat as used herein and in the appended claims, the singular forms “a,”“an,” and “the” include plural reference unless the context clearlydictates otherwise.

According to embodiments of the invention, amidoxime amino acid esters(I), N-hydroxyguanidine amino acid esters (II) or amidoxime prolinylesters of the formula (III) are proposed to be used:

wherein R1 is hydrogen, an alkyl radical, an aryl radical, one of thegroups listed in the following

or the salts thereof, as a substitute for one or more amidine functions,N-hydroxyamidine (amidoxime) functions, guanidine functions orN-hydroxyguanidine functions of a medicinal substance in drugs forimproving the solubility, bioavailability and enabling the medicinalsubstance to pass the blood-brain barrier.

N-hydroxyamidines (amidoximes) and N-hydroxyguanidines are successfulprodrug principles for increasing the oral bioavailability of amidines(DE 10 2006 034 256.9).

The esterification of the amidoximes or N-hydroxyguanidines with aminoacids proposed according to the invention, considerably improves thesolubility, in particular the solubility in aqueous media, and thebioavailability as compared to known prodrugs. The particular advantageof the water-soluble amino acid esters according to the invention istheir improved resorption from the gastrointestinal tract by specificamino acid and peptide transporters. A further advantage resides in thefact that, due to the esterification of the amidoximes orN-hydroxyguanidines with amino acids, injectable dosage forms arepossible again, since the water solubility is restored just as in thecase of amidines. The novel prodrug principles according to theinvention provide the possibility of passing the blood-brain barrier.This would be a decisive progress in the therapy of the African sleepingsickness, since the 2^(nd) phase of the disease can thus also be treatedefficiently.

According to the use according to the invention, the substituting of atleast one or more amidine functions, N-hydroxyamidine (amidoxime)functions, guanidine functions or N-hydroxyguanidine functions by theamidoxime amino acid ester and N-hydroxyguanidine amino acid esterachieves for the solubility of the medicinal substance concerned to beincreased. As a result, it can be firstly resorbed effectively afteroral administration, and subsequently be reconverted again into theactual active form, the amidine, respectively, guanidine, by the body'sown reductases and N-reductases (prodrug principle). The excellentresorbability of the modified amidoxime function or N-hydroxyguanidinefunction in the gastrointestinal tract is obviously due to the increasedsolubility of the active agent molecules. Furthermore, the novel prodrugprinciples are capable of enabling the blood-brain barrier to beovercome.

It is sufficient for the active agent to contain at least one or moreactive amidine functions, N-hydroxyamidine (amidoxime) functions,guanidine functions or N-hydroxyguanidine functions in the proposedform. The active agent consequently may contain, e.g., a plurality ofamidoxime functions (e.g. two as in the case of pentoxime ester) orN-hydroxyguanidine functions, with at least one of these groups beingthen modified in the manner described above. Mixtures of active agentsmay be used just the same, provided that at least one active agent hasone or more amidine functions, N-hydroxyamidine (amidoxime) functions,guanidine functions or N-hydroxyguanidine functions. The oral dosageform may be a liquid, semisolid or solid preparation, packaged inparticular as a tablet, dragee, pellet or microcapsule. In this case,the active agent or active agent mixture for embodiments of this type,in which liquid preparations are used, is incorporated in a suitablenon-toxic solvent such as water, monovalent alcohols, in particularethanols, polyvalent alcohols, in particular glycerine and/orpropanediol, polyglycols, in particular polyethylene glycols and/ormiglyol, glycerol formal, dimethyl isosorbite, natural or syntheticoils. For producing semisolid or solid preparations, the usual basematerials are used such as bentonite, veegum, guar flour and/orcellulose derivatives, in particular methylcellulose and/orcarboxymethyl cellulose, as well as polymers of vinyl alcohols and/orvinylpyrrolidones, alginates, pectins, polyacrylates, solid and/orliquid polyethylene glycols, paraffins, fatty alcohols, vaselines and/orwaxes, fatty acids and/or fatty acid esters.

Moreover, in solid preparations, the extenders known per se, such ascolloidal silicic acid, talc, lactose, starch powder, sugar, gelatine,metal oxides and/or metal salts may be contained. Stabilizers,emulsifiers, deflocculants and preservatives are suitable as furtheradditives.

The medicinal substances modified according to the use according to theinvention exhibit an excellent resorbability and thus bioavailability inoral administration, whereby the pharmacological effect of the amidineor guanidine is distinctly increased. As a result, an optimum dosageform for the oral application of amidines may be provided.

The use according to the invention gains particular importance in thatthe functional groups amidine and guanidine are essential constituentsof various important active agents for different fields of application.Inter alia, they are a constituent of the following substance classes,respectively active agents: protease inhibitors (thrombin inhibitorssuch as Melagatran, inhibitors of factor Xa, Factor VII, respectively ofall of the proteases of the coagulation cascade; matriptase inhibitors),anticoagulants, thrombolytics, antifibrinolytics, DNA-intercalating andRNA-intercalating compounds (such as pentamidine, diminazene,isometamidium), N-methyl-D-aspartate receptor antagonists and inhibitorsof viral enzymes (such as, e.g., neuraminidase inhibitors).

Active agents containing an effective amidine function or guanidinefunction may be used inter alia for inhibiting blood coagulation, forthe prophylaxis and therapy of visceral and cutaneous leishmaniasis,trypanosomiasis (African sleeping sickness), pneumonia caused bypneumocystis carinii (PCP), for inhibiting the growth of malign tumors,blood pressure reduction, neuroprotection, and for combating viralinfections such as influenza and HIV infections.

The listings above are merely exemplary, and the invention basicallyencompasses any active agents, which have at least one amidine functionor guanidine function that has been transferred according to theinvention into an improved prodrug. The method according to theinvention is thus applicable to a very wide range of substance classesand indications and is capable of distinctly increasing thebioavailability of many medicinal substances, the active form of whichcontains an amidine or a guanidine.

As examples for compounds modified according to the method according tothe invention, valbenzamidoxime, pentamidine valine ester and diminazenevaline ester can be mentioned.

The preparation and use according to the invention is explained in moredetail by means of exemplary embodiments, which are in no way intendedto limit the scope of the present invention.

Exemplary Embodiments Material and Methods

valbenzamidoxime1.21 g of benzamidoxime was dissolved in dried acetonitrile. Afteraddition of 2.604 g of tert-butoxycarbonyl valine, 154 mg of 4-dimethylaminopyridine and 2.988 g of dicyclohexyl carbodiimide, the mixture wasstirred at room temperature for three days. The completeness of thereaction was checked by means of DC.

The solvent was withdrawn and the raw product caused to crystallize froma mixture of ethanol/water (3/1, V/V).

Yield: 86%

Melting point: 147° C.

IR (KBr):

V=3496, 2928, 2850, 1744, 1686, 1612, 1392, 1374 cm⁻¹.

¹H-NMR (300 MHz, DMSO-d₆):

δ/ppm (TMS)=0.83 (d, 6H, ³J=6.4 Hz, CH(CH ₃)₂), 1.40 (s, 9H, C(CH₃)₃),2.08 (m, 1H, CH(CH₃)₂), 4.05 (t, 1H, ³J=8.0 Hz, CH—NH), 6.85 (s, 2H,NH₂), 7.31 (d, 1H, ³J=9.2 Hz, NH), 7.44 (m, 3H, Ar—H), 7.52 (m, 2H,Ar—H).

¹³C-NMR (75 MHz, DMSO-d₆):

δ/ppm (TMS)=18.25 (CH₃), 18.96 (CH₃), 28.15 (C(CH₃)₃), 30.13 (CH(CH₃)₂),59.09 (CH—NH), 78.29 (C(CH₃)₃), 126.68 (C_(Ar) 2.6), 128.30 (C_(Ar)3.5), 130.46 (C_(Ar) 1), 131.44 (C_(Ar) 4), 155.73 (C═N), 156.96 (C═O),168.88 (C═O).

MS (ESI):

m/z (%)=693 [2M+Na⁺] (15), 336 [M+H⁺] (100), 280 [M—(CH₃)₂C═CH₂+H⁺](44), 137 [BAO+H⁺] (31), 119 [Val+H⁺] (5).

C₁₇H₂₅N₃O₄ (335.41):

Ber. N 12.54% C 60.90% H 7.51% Gef. N 12.63% C 60.96% H 7.71%

For splitting-off the protecting group, 300 mg of thebocvalbenzamidoxime was dissolved in dioxane. HCl gas was introducedover a period of five minutes and stirred overnight. The free ester wasprecipitated by adding cold ethyl acetate and subsequently washedseveral times with ether.

Yield: 30%

IR (KBr):

V=3238, 3052, 2910, 2744, 1800, 1682, 1600, 1374 cm⁻¹.

¹H-NMR (300 MHz, DMSO-d₆):

δ/ppm (TMS)=1.02 (d, 3H, ³J=4.7 Hz, CH₃), 1.04 (d, 3H, ³J=4.7 Hz, CH₃),2.02 (m, 1H, CH(CH₃)₂), 3.94 (t, 1H, ³J=5.4 Hz, CH—NH₂), 7.22 (s, 2H,NH₂), 7.48 (m, 3H, Ar—H), 7.72 (m, 2H, Ar—H), 8.80 (s, 3H, NH₃), 9.03(s, 1H, NH).

¹³C-NMR (75 MHz, DMSO-d₆):

δ/ppm (TMS)=18.07 (CH₃(CH₃)₂), 29.38 (CH(CH₃)₂), 57.24 (CH—NH₂), 126.77(C_(Ar) 2.6), 128.33 (C_(Ar) 3.5), 130.62 (C_(Ar) 1), 130.09 (C_(Ar) 4),157.62 (C═N). 165.40 (C═O).

MS (ESI):

m/z (%)=493 [2M+Na⁺] (43), 471 [2M+H⁺] (42), 236 [M+H⁺] (100), 137[BAO+H⁺] (68), 121 [BA+H⁺] (20), 119 [BAO—H₂O+H⁺] (38), 118 [Val+H⁺](49).

C₁₂H₁₉N₃O₂Cl₂ (308.21):

Ber. C 46.76% H 6.21% N 13.63% Gef. C 46.54% H 6.25% N 13.12%

For proving the resorption from the gastrointestinal tract and thesubsequent reduction to benzamidine, the valbenamidoxime is orallyadministered to three rats. The metabolization of the ester intobenzamidine in this case takes place in vivo as follows:

Methods Related to the Execution of the Study with Rats

The animal study was permitted by the Schleswig-Holstein Ministry ofAgriculture, Environment and Rural Spaces on Jul. 4, 2007.

The anaesthesia was carried out using xylazine and ketamine. Both wereadministered by intramuscular injection. The silicone catheters wereimplanted in the vena jugularis and the arteria carotis. They havelocally antithrombotic and anti-inflammatory properties, but are notsystemically active. During the surgery, the eyes were protected with acornea-protective ointment (Oculotect®), and 3-4 ml of Ringer lactatesolution was applied subcutaneously for improving the postoperativeenergy supply. The animals were treated antiphlogistically (Finadyne®, 1mg/kg of body weight) and antibiotically (Amoxicillin® 15%, 10 mg/kg ofbody weight) and postoperatively attended and kept warm until they wokeup. The day after the surgery, the animals got Nutri Plus®, an energypaste (soy bean oil, molasses, cod-liver oil, meat extract, mineralpremixture, vitamin premixture).

After the test was completed, the animals were euthanized usingpentobarbital (i.v.).

Keeping of the Rats

Male Wistar rats having an average weight of 200 g served as the testanimals. The animals were kept individually in cages. Every second daythey got concentrated food. Water and dry food was available ad libitum.

Application of the Substances

For being able to determine the accurate dosage of the substances, theanimals were weighed the evening before the substance application. Thesubstances (prodrugs) to be administered orally were applied via astomach tube. For this purpose, valbenzamidoxime was solved in 100 mMphosphate buffer, pH 6.5. The intravenously administered benzamidine wassolved in 0.9% NaCl solution, so as to prevent haemolysis. After theinjection, rerinsing with at least 0.5 ml of 0.9% NaCl solution wascarried out. The substance application in each case took place in themorning.

The prodrugs were administered to three rats. Benzamidine was appliedintravenously to two rats. The orally administered doses ofvalbenzamidoxime were 50 mg/kg of body weight. Benzamidine was appliedin a concentration of 10 mg/kg of body weight.

Blood Sampling

Six blood samples can be taken from one rat. The test period for onecondition is one day. The blood samples were obtained over a period ofeight hours after oral application, respectively six hours afterintravenous application. After the oral administration, the samplingtook place after 30, 60, 90, 120, 240 and 480 minutes, after intravenousapplication after 5, 10, 20, 40, 80 and 360 minutes. Prior to the bloodwithdrawal, the catheter was emptied by a short aspiration until bloodappeared. The blood withdrawal (300 μl) was carried out by means ofMultivetten (Multivetten® 600, Sahrstedt, Nümbrecht). For keeping thecatheter clear, about 0.3 ml of a mixture of heparin and NaCl weresubsequently injected. The obtained full blood was centrifuged (1500 g,10 min, 4° C.). After the centrifugation, about 150 μl plasma was takenas a supernatant, pipetted into Eppendorf cups and frozen at −80° C.

Reprocessing of the Blood Samples

After slowly defrosting, 150 μl of plasma was diluted with Aqua bidest.ad 600 μl. The plasma samples were subsequently reprocessed by means ofsolid phase extraction. After conditioning the column with 1000 μl ofmethanol and equilibrating with 1000 μl of Aqua bidest., the sampleapplication (600 μl) was carried out. The sorbent was washed after thesample application with 600 μl of Aqua bidest. The elution of thesubstances was carried out by means of Aqua bidest., pH 3/methanol (6/4,V/V). Thereupon, the eluate is concentrated to dryness and absorbed with100 μl of Aqua bidest./methanol (9/1, V/V) and transferred to the HPLC.

HPLC Analysis for Separating Benzamidoxime and Benzamidine

For separating the substances to be analyzed, the following HPLC methodwas used:

HPLC pump: Waters 600 Detector: Waters 2417 Tunable Absorbance DetectorAutosampler: Waters 717 plus Autosampler Integrator: EZChrom ™ EliteClient/Server Version 2.8.3 Build 2249 recording and evaluation softwarestationary phase: Synergy Max-RP 80A; 250* 4.6 mm with precolumn C 184.0* 3.0 mm (Phenomenex, Aschaffenburg) Column temperature: 24° C.constant, by means of column heater Mobile phase: 10 mM of octylsulfonate in Aqua bidest., pH 2.5 (with conc. H₃PO₄)/acetonitrile(82.5/17.5, V/V) Run time: 30 minutes Detection: UV detector, 229 nmFlow rate: 1.0 ml/min Injection volume: 10 μl Detector sensitivity:absorbance units fullscale: 2,000 Retention times: benzamidoxime: 23.5 ±0.5 min benzamidine: 26.5 ± 0.5 min

The eluant was filtered using a Satorius membrane filter (0.45 μm) anddegassed in an ultrasonic bath for 15 minutes.

HPLC Analysis for Analyzing the Valbenzamidoxime, Benzamidoxime andBenzamidine

For separating the substances to be analyzed, the following HPLC methodwas used:

HPLC pump: Waters 600 Detector: Waters 2417 Tunable Absorbance DetectorAutosampler: Waters 717 plus Autosampler Integrator: EZChrom ™ EliteClient/Server Version 2.8.3 Build 2249 recording and evaluation softwarestationary phase: RP Select B with precolumn 4 * 4 (Merck KgaA,Darmstadt) Mobile phase: A: 25 mM ammonium acetate in Aqua bidest., pH6.3/acetonitrile (92.5/7.5, V/V) B: acetonitrile

time [min] A [%] B [%]  0-15 100 0 15-35 80 20 35-48 100 0

Run time: 48 minutes Detection: 229 nm Flow rate: 1.0 ml/min Injectionvolume: 10 μl Detector sensitivity: absorbance units fullscale: 2,000Retention times: benzamidine: 7.5 ± 0.3 min benzamidoxime: 12.0 ± 0.3min valbenzamidoxime: 32.0 ± 0.5 min

The eluant was filtered using a Satorius membrane filter (0.45 μm) anddegassed in an ultrasonic bath for 15 minutes.

The results of the tests are shown in FIG. 1, in which the plasma levelcurves of benzamidine after oral administration of the valbenzamidoximeare illustrated.

The metabolism of the diminazene valine ester is illustrated in theflowchart in FIG. 2, and the metabolism of the pentamidine valine esterin the flowchart in FIG. 3.

The oral bioavailability of the benzamidine after oral administration ofthe valbenzamidoxime could be determined from the obtained data (Table1):

TABLE 1 bioavailability of the benzamidine after oral administration ofvalbenzamidoxime bioavailability mean value standard deviation [%] [%][%] Rat 10 111.8 87.7 27.0 Rat 11 74.5 Rat 12 69.8

As can be seen from the Table above, benzamidine has a bioavailabilityof 88% after oral administration of the valbenzamidoxime. Thisdemonstrates that the prodrug was completely resorbed after oraladministration and metabolized into the active form benzamidine.

Particular embodiments of the invention are:

1. Use of an amidoxime amino acid ester of the formula (I) or anN-hydroxyguanidine amino acid ester of the formula (II) or amidoximeprolinyl ester of the formula (III)

wherein R1 is selected from the group consisting of hydrogen, an alkylradical, an aryl radical, and one of the groups illustrated in thefollowing

and the salts thereof, as a substitute for one or more amidinefunctions, N-hydroxyamidine (amidoxime) functions, guanidine functionsor N-hydroxyguanidine functions of a medicinal substance in drugs forimproving the solubility, bioavailability and/or capacity of themedicinal substance to pass the blood-brain barrier.

2. Use according to embodiment 1, characterized in that the medicinalsubstance is selected from the group of protease inhibitors,DNA-intercalating and RNA-intercalating compounds, inhibitors of viralenzymes and N-methyl-D-aspartate receptor antagonists.

3. Use according to embodiment 2, characterized in that the proteaseinhibitor is a thrombin inhibitor, an inhibitor of factor Xa, FactorVII, or of all of the proteases of the coagulation cascade, or amatriptase inhibitor.

4. Use according to embodiment 2, characterized in that the proteaseinhibitor is an urokinase inhibitor.

5. Use according to embodiment 2, characterized in that theDNA-intercalating or RNA-intercalating compound is pentamidine,diminazene or isometamidium.

6. Use according to embodiment 2, characterized in that the inhibitor ofviral enzymes is a neuraminidase inhibitor.

7. Use according to any one of the preceding embodiments, characterizedin that the medicinal substance is designed for the prophylaxis andtherapy of visceral and/or cutaneous leishmaniasis, trypanosomiasis, the2nd phase of trypanosomiasis, or pneumonia caused by pneumocystiscarinii, for inhibiting the growth of malign tumors, for inhibitingblood coagulation, for blood pressure reduction, for neuroprotection,and for combating viral infections including influenza and HIVinfections.

It will be appreciated by those skilled in the art that changes could bemade to the embodiments described above without departing from the broadinventive concept thereof. It is understood, therefore, that thisinvention is not limited to the particular embodiments disclosed, but itis intended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

We claim:
 1. A method of producing a prodrug for a medicinal substance,comprising using a partial structure of formula (I), formula (II) orformula (III),

as a substitute for at least one of an amidine function in the medicinalsubstance to obtain the prodrug for the medicinal substance, wherein R1is selected from the group consisting of hydrogen, an alkyl radical, anaryl radical, and one selected from the group consisting of thefollowing:

and the salts thereof, wherein the medicinal substance is selected fromthe group consisting of benzamidine, pentamidine, diminazene,isometamidium and melagatran.
 2. The method according to claim 1,wherein the prodrug is selected from the group consisting ofvalbenzamidoxime, pentamidine valine ester and diminazene valine ester.3. The method according to claim 1, wherein the overall structure of theprodrug comprises a plurality of at least one of the partial structuresof formulas (I), (II) and (III).
 4. The method according to claim 1,wherein the medicinal substance is configured for the prophylaxis andtherapy of visceral and/or cutaneous leishmaniasis, trypanosomiasis, the2^(nd) phase of trypanosomiasis, or pneumonia caused by pneumocystiscarinii, for inhibiting the growth of malign tumors, for inhibitingblood coagulation, for blood pressure reduction, for neuroprotection, orfor combating viral infections.
 5. The method according to claim 1,wherein the prodrug is pentamidine valine ester.
 6. A prodrug for amedicinal substance, the prodrug comprising a partial structure havingthe formula (I), (II) or (III),

wherein the formula (I), (II) or (III) is used as a substitute for atleast one of an amidine function in the medicinal substance, R1 isselected from the group consisting of hydrogen, an alkyl radical, anaryl radical, and one selected from the group consisting of thefollowing:

and the salts thereof, and the medicinal substance is selected from thegroup consisting of benzamidine, pentamidine, diminazene, isometamidiumand melagatran.
 7. The prodrug according to claim 6, wherein the prodrugis selected from the group consisting of valbenzamidoxime, pentamidinevaline ester and diminazene valine ester.
 8. The prodrug according toclaim 6, wherein the medicinal substance is configured for theprophylaxis and therapy of visceral and/or cutaneous leishmaniasis,trypanosomiasis, the 2^(nd) phase of trypanosomiasis, or pneumoniacaused by pneumocystis carinii, for inhibiting the growth of maligntumors, for inhibiting blood coagulation, for blood pressure reduction,for neuroprotection, or for combating viral infections.
 9. The prodrugaccording to claim 6, wherein the overall structure of the prodrugcomprises a plurality of at least one of the partial structures offormulas (I), (II) and (III).
 10. The prodrug according to claim 6,wherein the prodrug is pentamidine valine ester.